Sharing resources fairly can be a tricky business. Is it better to dish out more of something good, such as food or money, even if only some people receive this benefit, or is it more important to ensure that everyone receives at least some gain, even if the total goodness that’s dolled out is actually lower?

Ming Hsu and colleagues scanned the brains of 26 participants as they made decisions like this between efficiency on the one hand, and equity on the other.

The researchers had donated a certain amount of money to orphans in Uganda but unfortunately some of this money had to be withdrawn. In a series of choices, participants had to say whether they wanted a smaller sum to be withdrawn, with the downside that it would all be taken from just one child, or if instead they’d prefer a larger amount overall to be withdrawn, with the advantage that the burden of loss would be shared between two children.

Consistent with past research into similar moral decisions, the participants showed a bias for choosing equity (that is, a larger amount withdrawn, but with the burden shared) over efficiency (less withdrawn, but all from one kid).

What’s new is that the researchers were able to determine that an emotion-related region called the insula was responsible for encoding the equity of each option, whilst a reward-based region called the putamen was involved in encoding efficiency. In fact, differences in how sensitive each participant was to these two concerns was reflected in their levels of brain activity in these two regions.

The findings provide a biological perspective on age-old philosophical questions about distributive justice, and appear to support the view of thinkers such as David Hume and Adam Smith who argued that emotions play a fundamental role in moral decisions of this kind._________________________________